AQA AS Chemistry

Revision Notes

1.6.7 Bond Enthalpies

Bond Enthalpies

  • The amount of energy required to break one mole of a specific covalent bond in the gas phase is called the bond dissociation energy
  • Bond dissociation energy (E) is usually just simplified to bond energy or bond enthalpy
  • In symbols, the type of bond broken is written in brackets after E
    • Eg. E (H-H) is the bond energy of a mole of single bonds between two hydrogen atoms

Average bond energy

  • Bond energies are affected by other atoms in the molecule (the environment)
  • Therefore, an average of a number of the same type of bond but in different environments is calculated
  • This bond energy is known as the average bond energy
  • Since bond energies cannot be determined directly, enthalpy cycles are used to calculate the average bond energy

Bond Energies images, downloadable AS & A Level Chemistry revision notes

Bond energies are affected by other atoms in the molecule, so average bond enthalpies are listed in data tables

Calculating enthalpy change from bond energies

  • Bond energies are used to find the ΔHr of a reaction when this cannot be done experimentally
  • The formula is:

Chemical Energetics Equation Enthalpy Change of Reaction using Bond, downloadable AS & A Level Chemistry revision notes

The formula for calculating the standard enthalpy change of reaction using bond energies

Worked Example

Calculating the enthalpy change in the Haber process

Calculate the change in enthalpy of reaction for the Haber process, producing ammonia from hydrogen and nitrogen:

N2 (g) + 3H2 (g) ⇌ 2NH3 (g)

The relevant bond energies are given in the table below:

Answer

Step 1: Use the equation to work out the bonds broken and formed and set out the calculation as a balance sheet as shown below:

N2 (g) + 3H2 (g) ⇌ 2NH3 (g)

Chemical Energetics Table 2_5, downloadable AS & A Level Chemistry revision notes

Note! Values for bonds broken are positive (endothermic) and values for bonds formed are negative (exothermic)

Step 2: Calculate the standard enthalpy of reaction

ΔHr = enthalpy change for bonds broken + enthalpy change for bonds formed

= (+2253 kJ mol-1) + (-2346 kJ mol-1)

= -93 kJ mol-1

Worked Example

Calculating the enthalpy of combustion using bond enthalpies

The complete combustion of ethyne, C2H2, is shown in the equation below:

Using the average bond enthalpies given in the table, what is the enthalpy of combustion of ethyne?

Bond Enthalpy - Worked Example 2 Data table, downloadable IB Chemistry revision notes

Answer

Step 1: The enthalpy of combustion is the enthalpy change when one mole of a substance reacts in excess oxygen to produce water and carbon dioxide

The chemical reaction should be therefore simplified such that only one mole of ethyne reacts in excess oxygen:

H-C≡C-H + 2 ½ O=O → H-O-H + 2O=C=O

Step 2: Set out the calculation as a balance sheet as shown below:

Bond Enthalpy - Worked Example 2 Answer, downloadable IB Chemistry revision notes

ΔHr = enthalpy change for bonds broken + enthalpy change for bonds formed

= (+2912 kJ mol-1) + (- 4142 kJ mol-1)

= -1230 kJ mol-1

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